1. Field of the Invention
The present invention relates generally to high temperature polymers. It relates particularly to a semi-interpenetrating polymer network approach to the obtainment of tougher and more microcracking resistant high temperature polymers.
2. Description of the Related Art
For some time, there has been an active search for an improved high temperature matrix resin capable of performing at 316.degree. C. in air for several hundreds of hours. Some of the desired properties for this improved material are significantly improved toughness and microcracking resistance, excellent processing capability and mechanical performance, and cost effectiveness.
To develop such an improved material, the approach used in early studies concentrated on the synthesis of flexibilized PMR polyimides, in which flexibilizing linking groups, such as ether and/or hexafluoroisopropylidene groups, were incorporated into the PMR polymer chain. (A PMR polyimide contains a nadic end group acting as a crosslinking site, and is processed by the polymerization of monomer reactant (PMR) process as set forth in U.S. Pat. No. 3,745,149). Vannucci and Bowles (R. D. Vannucci and K. J. Bowles, Proceedings of the 17th Nat. SAMPE Technical Conference, 17 352 (1985)) reported a small increase (35 percent) in composite impact energy for the flexibilized PMR-polyimide compared to a PMR polyimide designated PMR-15. (See infra for details concerning the synthesis of PMR-15.) This improvement in toughness, however, was achieved with a compromise in lowering the glass transition temperature. Similar findings were also reported by Delvigs (P. Delvigs, Polymer Composites, 7(2), 101 (1986). These results show clearly that such an approach is not effective in providing adequate toughness for the improved material.
St. Clair et al (U.S. Pat. No. 4,695,610) and others have synthesized semi-interpenetrating network (semi-IPN) polyimides from easy-to-process, but brittle thermosetting polyimides and tough, but difficult-to-process thermoplastic polyimides. However, none of these prior art products have the desired combination of properties set forth hereinabove.